The invention relates generally to industrial computed tomography (CT) systems, and more specifically to scatter correction in computed tomography of industrial parts.
Scatter is a deflection in radiation (suitable for imaging, such as X-rays) direction caused by certain interactions of the radiation within a target material, also referred to as an object. The phenomenon is significant in computed tomography of industrial parts because the materials, often metallic in nature, scatter x-rays to a greater degree, and is especially deleterious for three-dimensional, volumetric computed tomography where the entire object is irradiated by a cone beam of x-rays. This spatially-varying background adds to the true signal and can produce pronounced artifacts when the three-dimensional image of the object is mathematically reconstructed.
The primary measurement data in computed tomography are sets of x-ray projections taken from various angles with respect to the object. In what follows, it is assumed that a complete computed tomography system having a computer is available, that the distribution of x-ray intensity in the various projection views has been detected, measured, and digitized in some manner known to the art to obtain raw projection data, which numerically represent the projection view of the object. These arrays of numbers representing the various projection views are accessible for numerical operation by the computer. It is further assumed that the projection arrays are afterwards combined and processed according to the known methods of computed tomography (CT) to produce a two-dimensional (2D) or a three-dimensional (3D) x-ray attenuation map, or a 2D or 3D representation, of the object. Typically generation of a single 2D image in a single reconstruction step is referred to as planar CT and generation of a 3D image in a single reconstruction step is referred to as volumetric or cone-beam CT. Further a series of 2D images with appropriate step between adjacent slide locations is comparable to the 3D image set produced in a single cone-beam reconstruction. Typically, planar CT systems utilize a linear detector array and cone-beam CT systems utilize an area detector array.
Hereinafter, reference to a 3D image will include by implication reference to a 2D image as a subset of a 3D image. Further, the computer is configured to provide the means for the reconstruction of and analysis of a voxellized representation of the object.
It is known that the image artifacts caused by scattered x-rays falling on the various projection views can be corrected if the fraction of total signal at each point of every projection caused by scatter is estimated and then digitally subtracted before the projections are combined in the image reconstruction step.
Until now, approaches for estimating this scattered component include making ancillary measurements using a series of x-ray blocking slits of varying width placed between the object and the x-ray detector. The rationale is that the scattered signal, being incident from a range of directions, can be estimated by extrapolating the series of slit measurements to zero width. However, such a method requires extensive added hardware and provides only a coarse grid of scatter estimates. More importantly, this approach has proven experimentally difficult and unable to provide accurate scatter estimates.
A different approach has involved calculation of the scattered signal from physical first principles using prior knowledge of the object geometry. Accurate scatter estimates may be possible in this way using Monte Carlo radiation transport computer codes. However such estimates are calculation heavy and consume a large amount of processor time, a requirement which is prohibitive where a variety of different complex shape are to be imaged, as is the case in industrial imaging.
It would therefore be desirable to have methods and systems that provide substantially accurate scatter correction estimates, and provide advantage in terms of computation time.